The present invention relates to a liquid crystal display using a TFT, and more particularly to a method of driving a liquid crystal.
FIG. 1 shows the structure of a conventional liquid crystal display (LCD) using a Thin Film Transistor (TFT). A thin film transistor element 1, a source line 2, a gate line 3, a drain 4 and a pixel electrode 5 are formed on a glass substrate 6 to form a TFT substrate. A counter electrode 7 is formed on a glass substrate 8 to form an opposite substrate. The TFT substrate and the opposed substrate are provided in parallel with each other and a liquid crystal is interposed between the TFT substrate and the opposite substrate.
FIG. 2 shows an equivalent circuit for one pixel of FIG. 1.
In FIG. 2, the reference numeral 9 denotes a source signal to be applied to the source line 2 and the reference numeral 10 denotes a gate signal to be applied to the gate line 3. A symbol Cgd represents a coupling capacitance between a gate and a drain, a symbol Cds represents a coupling capacitance between a source and a drain, and a symbol C1c represents a coupling capacitance of a liquid crystal interposed between a pixel electrode and a counter electrode. Cs represents a retaining capacitance formed to enhance a retaining characteristic of a pixel and to improve a picture quality.
FIG. 3 shows a waveform of a signal to be applied to a pixel.
The source signal 9 is an alternating voltage having an amplitude Vsa in which a central electric potential Vso is a median. The amplitude Vsa corresponds to a gradation to be displayed on a pixel. The gate signal 10 is set to the High level (hereinafter referred to as xe2x80x9cHixe2x80x9d level) only for one scanning period and to the Low level (hereinafter referred to as xe2x80x9cLoxe2x80x9d level) for other periods. The reference numeral 11 denotes a waveform representing an electric potential of the pixel electrode 5.
First of all, when the gate signal 10 becomes the Hi level in an odd frame 101 shown in FIG. 3, the electric potential 11 of the pixel electrode 5 becomes have the level of the source signal 9. When the gate signal 10 becomes the Lo level, a voltage drop of xcex94Vgd is generated on the electric potential 11 of the pixel electrode 5 under the influence of the coupling capacitance Cgd between the gate and the drain. The voltage drop xcex94Vgd is referred to as a feed-through voltage and is expressed by the following equation (1):
xcex94Vgd=xcex94Vgxc3x97Cgd/(C1c+Cgd+Cds+Cs)xe2x80x83xe2x80x83Equation (1)
wherein xcex94Vg represents an amount of voltage change of the gate signal 10.
Then, the electric potential 11 of the pixel electrode 5 is held mainly by the retaining capacitance Cs for one frame.
When the gate signal 10 becomes the Hi level again in a subsequent even frame 102, the electric potential 11 of the pixel 5 becomes the level of the source signal 9. When the gate signal 10 becomes the Lo level, a voltage drop xcex94Vgd is also generated. As described above, the voltage drop xcex94Vgd is expressed by the equation (1).
On the other hand, a one-dotted chain line 12 shown in FIG. 3 indicates an electric potential of the counter electrode 7, which is generally referred to as a common signal. An electric potential of the common signal 12 can be usually regulated by a variable resistor or the like which is additionally provided, and the absolute values of a voltage Vo to be applied to the liquid crystal in the odd frame 101 and a voltage Ve to be applied to the liquid crystal in the even frame 102 are set to be equal to each other. At that time, the electric potential of the common signal is referred to as an optimum Vcom.
In the LCD using a TFT method, generally, the writing of positive and negative polarities is carried out at a frequency of approximately 60 Hz. Accordingly, in the case in which the absolute values of the voltage Vo to be applied to the liquid crystal in the odd frame and the voltage Ve to be applied to the liquid crystal in the even frame are not equal to each other, so-called a flicker having a frequency of approximately 30 Hz is observed.
Furthermore, in the case in which the absolute values of the voltages Vo and Ve are not set to be equal to each other, the magnitudes of the alternating voltages to be applied to the liquid crystal are not equal to each other with positive and negative polarities. As a result, a DC voltage is applied. At that time, as shown in FIG. 4, an electric charge is moved in a direction of each electrode through the DC voltage applied to a liquid crystal layer.
When the same image is displayed for a long time in the LCD and another image is then displayed, a xe2x80x9cstickingxe2x80x9d is caused, in which a residual DC is generated and a last image remains as an afterimage.
In order to prevent xe2x80x9cstickingxe2x80x9d from causing, accordingly, the electric potential of the common signal 12 is regulated to coincide with the center of the electric potential 11 of the pixel electrode 5.
However, the coupling capacitance C1c caused by the liquid crystal in the components of the equation (1) has a dependency on an applied voltage. FIG. 5 shows the relationship between a voltage applied to the liquid crystal and the coupling capacitance C1c caused by the liquid crystal. An axis of abscissa indicates an amplitude Vsa of the source signal 9 as the voltage to be applied to the liquid crystal and an axis of ordinate indicates the coupling capacitance C1c caused by the liquid crystal. The value of the coupling capacitance C1c caused by the liquid crystal is varied depending on the voltage to be applied to the liquid crystal, that is, a gradation of an image to be displayed.
Accordingly, the feed-through voltage xcex94Vgd expressed in the equation (1) is not always constant but is changed as shown in FIG. 6 depending on the amplitude Vsa of the source signal 9, that is, the gradation of the image to be displayed.
As is apparent from FIG. 6, in the case in which the amplitude Vsa of the source signal 9 is great, that is, a gradation close to a black color is to be displayed, the feed-through voltage xcex94Vgd is low. In the case in which the amplitude Vsa of the source signal 9 is small, that is, a gradation close to a white color is to be displayed, the feed-through voltage xcex94Vgd is high.
In order to make the absolute values of the voltage Vo to be applied to the liquid crystal in the odd frame and the voltage Ve to be applied to the liquid crystal in the even frame to be equal to each other, it is necessary to set the electric potential of the common signal 12 to be low during a white display with a high feed-through voltage xcex94Vgd and to set the electric potential of the common signal 12 to be high during a black display with a low feed-through voltage xcex94Vgd. This relationship is shown in FIG. 7.
In FIG. 7, an axis of abscissa indicates the amplitude Vsa of the source signal 9, that is, a gradation of an image to be displayed, and an axis of ordinate indicates an optimum electric potential Vcom of the common signal. As is apparent from FIG. 7, the optimum electric potential Vcom of the common signal is varied every gradation. However, the counter electrode 7 to which the common signal 12 is to be applied is common over the whole region of a screen. Accordingly, when different gradations are displayed in the screen, there is always a pixel which is not given an optimum electric potential Vcom of the common signal and a DC voltage is applied to cause xe2x80x9cstickingxe2x80x9d.
In order to compensate for the feed-through voltage xcex94Vgd to be varied depending on the gradation, therefore, an offset compensation driving is used.
With reference to FIGS. 8 and 9, the principle of the offset compensation driving will be described. As mentioned above, if the amplitude Vsa of the source signal 9 is small, the feed-through voltage xcex94Vgd is high. Accordingly, as shown in FIG. 8, the central electric potential Vso of the source signal 9 is set to be high. On the other hand, if the amplitude Vsa of the source signal 9 is large, the feed-through voltage xcex94Vgd is low. Accordingly, it is preferable that there is no problem even if the central electric potential Vso of the source signal 9 is low.
By setting the central electric potential Vso of the source signal 9 as shown in FIG. 8, the electric potential Vcom of the common signal for causing the absolute values of the voltage Vo to be applied to the liquid crystal in the odd frame and the voltage Ve to be applied to the liquid crystal in the even frame to be equal to each other is almost unchanged over all the gradations as shown in FIG. 9. Accordingly, the electric potential of the common signal 12 to be applied to the counter electrode 7 is made coincident with the electric potential Vcom in FIG. 9. Consequently, also in the case in which a gradation to be varied in each region of a screen, there is no pixel to which the DC voltage is to be applied and the xe2x80x9cstickingxe2x80x9d is not caused.
In the case in which the offset compensation driving is to be used, an offset compensation value is set by selecting a position on a screen to obtain the optimum central electric potential Vso for each gradation in that position, that is, each amplitude Vsa of the source signal 9.
However, the optimum central electric potential Vso for each amplitude Vsa of the source signal 9 is varied depending on a position in the screen, which is considered to be caused by the following reasons.
(1) The waveform of the gate signal 10 is varied depending on a position in the screen. In the vicinity of an input section for the gate signal, the gate signal 10 has a signal waveform close to an ideal rectangular wave in which a rise and a fall are sharp. When a distance from the input section for the gate signal is increased, the signal waveform has a xe2x80x9croundedxe2x80x9d rise and fall. Accordingly, the value of xcex94Vg in the equation (1) is apparently reduced in a position kept apart from the input section for the gate signal. Therefore, the feed-through voltage xcex94Vgd is also varied in each position of the screen.
(2) In general, the retaining capacitance Cs has an uneven distribution depending on the position in the screen. Accordingly, the feed-through voltage xcex94Vgd expressed by the equation (1) is also varied in each position of the screen.
(3) The characteristic of the liquid crystal is not uniform over the whole screen. For this reason, the coupling capacitance C1c of the liquid crystal also has a distribution depending on the position in the screen. Accordingly, the feed-through voltage xcex94Vgd expressed by the equation (1) is also varied in each position of the screen.
For the above-mentioned reasons, the optimum central electric potential Vso for each amplitude Vsa of the source signal 9, that is, the offset compensation value is varied depending on the position in the screen. Accordingly, even if the offset compensation value is set in a certain position of the screen as in the prior art, the set value is not optimum in other positions. Therefore, the xe2x80x9cstickingxe2x80x9d is generated.
As a result of studying the xe2x80x9cstickingxe2x80x9d for various LCDs, the following conclusion has been drawn.
A first aspect of the present invention is directed to a method of driving a liquid crystal display, where said liquid crystal display comprising:
two substrates which are opposed to each other in such a manner that a liquid crystal layer is interposed between said two substrates;
a source line, to which source signals are fed, said source line being provided on one of said two substrates;
a gate line, to which gate signals are fed, said gate line being provided on said one of said two substrates;
a TFT element connected with said source line and said gate line;
a pixel electrode connected with a drain of said TFT element;
a counter electrode, to which common signals of direct or alternating current is applied, said counter electrode being provided on the other one of said two substrates;
wherein an amplitude of said source signals is varied so as to vary an electric potential of said pixel electrode, whereby a potential difference between said pixel electrode and said counter electrode is varied to change an alignment of liquid crystal molecules, so that gradation displayed on a pixel is controlled;
wherein a central electric potential of common signals applied on said counter electrode can be set to compensate a reduction of an electric potential of said pixel electrode induced by changing an electric potential of the gate signals;
wherein a central electric potential of said source signals can be varied every gradations to compensate a reduction of an electric potential induced by said gate signals which are different every gradations;
said method comprising steps of:
setting said central electric potential of said source signals and said central electric potential of said common signals, in such a manner as to compensate said reduction of the electric potential induced by said gate signals in a case of the gradation where said amplitude of said source signals is large, and setting said central electric potential of said source signals, in such a manner that said central electric potential of said source signals is higher than said central electric potential of said source signals compensating said reduction of the electric potential induced by said gate signals in case of the gradation where said amplitude of said source signals is small.
As a result, the xe2x80x9cstickingxe2x80x9d can be relieved over a wide range of a screen.
A second aspect of the present invention is directed to a method of driving a liquid crystal display, where said liquid crystal display comprising:
two substrates which are opposed to each other in such a manner that a liquid crystal layer is interposed between said two substrates;
a source line, to which source signals are fed, said source line being provided on one of said two substrates;
a gate line, to which gate signals are fed, said gate line being provided on said one of said two substrates;
a TFT element connected with said source line and said gate line;
a pixel electrode connected with a drain of said TFT element;
a counter electrode, to which common signals of direct or alternating current is applied, said counter electrode being provided on the other one of said two substrates;
wherein an amplitude of said source signals is varied so as to vary an electric potential of said pixel electrode, whereby a potential difference between said pixel electrode and said counter electrode is varied to change an alignment of liquid crystal molecules, so that gradation displayed on a pixel is controlled;
wherein a central electric potential of common signals applied on said counter electrode can be set to compensate a reduction of an electric potential of said pixel electrode induced by changing an electric potential of the gate signals;
wherein a central electric potential of said source signals can be varied every gradations to compensate a reduction of an electric potential induced by said gate signals which are different every gradations;
said method comprising steps of:
in a pixel where said reduction of an electric potential induced by said gate signals different every gradations is the largest, setting said central electric potential of said source signals and said central electric potential of said common signals, in such a manner as to compensate said reduction of the electric potential induced by said gate signals in a case of the gradation where said amplitude of said source signals is large, and setting said central electric potential of said source signals, in such a manner that said central electric potential of said source signals is higher than said central electric potential of said source signals compensating said reduction of the electric potential induced by said gate signals in case of the gradation where said amplitude of said source signals is small.
As a result, the xe2x80x9cstickingxe2x80x9d can be relieved over a wide range of a screen.
A third aspect of the present invention is directed to a method of driving a liquid crystal display, where said liquid crystal display comprising:
two substrates which are opposed to each other in such a manner that a liquid crystal layer is interposed between said two substrates;
a source line, to which source signals are fed, said source line being provided on one of said two substrates;
a gate line, to which gate signals are fed, said gate line being provided on said one of said two substrates;
a TFT element connected with said source line and said gate line;
a pixel electrode connected with a drain of said TFT element;
a counter electrode, to which common signals of direct or alternating current is applied, said counter electrode being provided on the other one of said two substrates;
wherein an amplitude of said source signals is varied so as to vary an electric potential of said pixel electrode, whereby a potential difference between said pixel electrode and said counter electrode is varied to change an alignment of liquid crystal molecules, so that gradation displayed on a pixel is controlled;
wherein a central electric potential of common signals applied on said counter electrode can be set to compensate a reduction of an electric potential of said pixel electrode induced by changing an electric potential of the gate signals;
wherein a central electric potential of said source signals can be varied every gradations to compensate a reduction of an electric potential induced by said gate signals which are different every gradations;
said method comprising steps of:
in all gradations, of which amplitude of said source signals is large or small is included, setting a central electric potential of said common signals to be smaller than that in combination of said central electric potential of said common signals with said central electric potential of said source signals to compensate said reduction of the electric potential due to the gate signal.
Consequently, the xe2x80x9cstickingxe2x80x9d can be reduced for a gradation having a small amplitude of the source signal and the xe2x80x9cstickingxe2x80x9d is not deteriorated for a gradation having a great amplitude of the source signal.
A fourth aspect of the present invention is directed to a method of driving a liquid crystal display, where said liquid crystal display comprising:
two substrates which are opposed to each other in such a manner that a liquid crystal layer is interposed between said two substrates;
a source line, to which source signals are fed, said source line being provided on one of said two substrates;
a gate line, to which gate signals are fed, said gate line being provided on said one of said two substrates;
a TFT element connected with said source line and said gate line;
a pixel electrode connected with a drain of said TFT element;
a counter electrode, to which common signals of direct or alternating current is applied, said counter electrode being provided on the other one of said two substrates;
wherein an amplitude of said source signals is varied so as to vary an electric potential of said pixel electrode, whereby a potential difference between said pixel electrode and said counter electrode is varied to change an alignment of liquid crystal molecules, so that gradation displayed on a pixel is controlled;
wherein a central electric potential of common signals applied on said counter electrode can be set to compensate a reduction of an electric potential of said pixel electrode induced by changing an electric potential of the gate signals;
wherein a central electric potential of said source signals can be varied every gradations to compensate a reduction of an electric potential induced by said gate signals which are different every gradations;
said method comprising steps of:
in all gradations, of which amplitude of said source signals is large or small is included, setting a central electric potential of said common signals to be smaller than that in combination of said central electric potential of said common signals with said central electric potential of said source signals to compensate said reduction of the electric potential due to the gate signals; and
setting a central electric potential of said source signals in gradations, of which amplitude of said source signals is small, to be higher than said combination.
Consequently, it is possible to more enhance the effect of reducing the xe2x80x9cstickingxe2x80x9d for the gradation having the small amplitude of the source signal.
A fifth aspect of the present invention is directed to a method of driving a liquid crystal display, where said liquid crystal display comprising:
two substrates which are opposed to each other in such a manner that a liquid crystal layer is interposed between said two substrates;
a source line, to which source signals are fed, said source line being provided on one of said two substrates;
a gate line, to which gate signals are fed, said gate line being provided on said one of said two substrates;
a TFT element connected with said source line and said gate line;
a pixel electrode connected with a drain of said TFT element;
a counter electrode, to which common signals of direct or alternating current is applied, said counter electrode being provided on the other one of said two substrates;
wherein an amplitude of said source signals is varied so as to vary an electric potential of said pixel electrode, whereby a potential difference between said pixel electrode and said counter electrode is varied to change an alignment of liquid crystal molecules, so that gradation displayed on a pixel is controlled;
wherein a central electric potential of common signals applied on said counter electrode can be set to compensate a reduction of an electric potential of said pixel electrode induced by changing an electric potential of the gate signals;
wherein a central electric potential of said source signals can be varied every gradations to compensate a reduction of an electric potential induced by said gate signals which are different every gradations;
said method comprising steps of:
in all gradations, of which amplitude of said source signals is large or small is included, setting a central electric potential of said common signals to be smaller than that in combination of said central electric potential of said common signals with said central electric potential of said source signals to compensate said reduction of the electric potential due to the gate signals;
setting a central electric potential of said source signals in gradations, of which amplitude of said source signals is small, to be higher than said combination; and
setting a central electric potential of said source signals in gradations, of which amplitude of said source signals is large, to be higher than said combination.
Consequently, the xe2x80x9cstickingxe2x80x9d can be reduced, and furthermore, display failures such as a flicker of a screen, a shot unevenness and the like are not caused.